Chemical transition of nitrate ions accompanied with corrosion of carbon steel under alkaline conditions

Honda, Akira; Kato, Takashi; Tateishi, Tsuyoshi*; Imakita, Tsuyoshi*; Masuda, Kaoru*; Kato, Osamu*; Nishimura, Tsutomu*

Migration of radioactive material can be affected by the redox condition and the concentration of ligands in the repository of radioactive waste. It is possible that radioactive waste contains nitrate which can affect the migration behavior of radioactive nuclides by both changing the redox condition of the environment and acting as a ligand. On the other hand, several researchers observed the reduction of nitrate ions in ammonia due to the iron. Ammonia has a potential to ligand for radioactive nuclides. Nitrate can also affect the rate of hydrogen gas evolution accompanied by metal corrosion through changing the rest potential of metal by its oxidizing nature. Carbon steel was, therefore, immersed in an aqueous solution of sodium nitrate in a closed system for observing both the chemical interaction between metal and nitrate, and the effect of nitrate on the hydrogen gas evolution rate. The experimental pH range of the solution was 10.0-13.5 which corresponds to the pH range of pore fluid of cementitious material. The cathodic current density shows a Tafel equation type potential dependency in the aqueous solution containing nitrate or nitrite. In spite of the acceleration of cathodic reaction due to the existence of nitrate, the corrosion rates of carbon steel were not accelerated in the nitrate solutions. This fact suggests that the system is controlled by the anodic reaction. The nitrate reduction accompanied by the corrosion of carbon steel is considered to be a series reaction such as nitrate nitrite ammonia. The nitrate reduction reaction compete with the water reduction reaction within the anodic controlled condition, therefore nitrate strongly reduced the hydrogen evolution rate. The generation rates of ammonia were independent of the concentration of nitrate.